Commercial aircraft typically include environmental control systems that regulate air supply, temperature, and cabin pressurization on the aircraft. Such environmental control systems may also be utilized to provide cooling to components of the aircraft itself, such as electrical systems. The environmental control system of most jet aircraft typically includes two or more air cycle machines, which provide pressurized air of an appropriate temperature to the cabin of the aircraft.
Air cycle machines typically operate using heated, pressurized bleed air that is diverted to the air cycle machines from the jet engines of the aircraft or from an auxiliary power unit of the aircraft. The air cycle machines typically also use ram air, which is ambient air that enters the aircraft through a ram scoop.
Typical air cycle machines include a turbine that is mechanically connected to a compressor, such that the turbine provides a rotational force to the compressor in order to operate the compressor. The turbine and compressor are selectively operable, and cooperate to reduce the temperature of the bleed air to allow cooling of the aircraft. Typical air cycle machines also include a heat exchanger that cools the bleed air using the ram air. Generally, the turbine and compressor are only used to cool the bleed air when the aircraft is on the ground, as the temperature of the ram air during flight is usually sufficient to cool the bleed air.
Diversion of the bleed air from the jet engines of the aircraft or from the auxiliary power unit of the aircraft accounts for a significant amount of fuel usage. It would be desirable to reduce the fuel usage attributable to operation of air cycle machines.